Brain excitability is defined as the level of arousal of an animal, a continuum that ranges from coma to convulsions, and is regulated by various neurotransmitters. In general, neurotransmitters are responsible for regulating the conductance of ions across neuronal membranes. At rest, the neuronal membrane possesses a potential (or membrane voltage) of approximately −70 mV, the cell interior being negative with respect to the cell exterior. The potential (voltage) is the result of ion (K+, Na+, Cl−, organic anions) balance across the neuronal semipermeable membrane. Neurotransmitters are stored in presynaptic vesicles and are released as a result of of neuronal action potentials. When released into the synaptic cleft, an excitatory chemical transmitter such as acetylcholine will cause membrane depolarization (change of potential from −70 mV to −50 mV). This effect is mediated by postsynaptic nicotinic receptors which are stimulated by acetylcholine to increase the membrane permeability of Na+ ions. The reduced membrane potential stimulates neuronal excitability in the form of a postsynaptic action potential.
NMDA receptors are highly expressed in the CNS and are involved in excitatory synaptic transmission and synaptic plasticity as well as excitotoxicity. These receptors are ligand-gated ion channels that admit Ca2+ after binding of the neurotransmitter glutamate and are fundamental to excitatory neurotransmission and normal CNS function. NMDA receptors are heteromeric complexes comprised of NR1, NR2, and/or NR3 subunits and possess distinct recognition sites for exogenous and endogenous ligands. These recognition sites include binding sites for glycine, and glutamate agonists and modulators. These modulators may be useful as therapeutic agents with potential clinical uses as cognitive enhancers and in the treatment of psychiatric disorders in which glutamatergic transmission is reduced or defective (see, e.g., Horak et al., J. of Neuroscience, 2004, 24(46), 10318-10325).
Neuroactive steroids such as pregnenolone sulfate (PS) have been shown to exert direct modulatory effects on several types of neurotransmitter receptors, such as GABAA, glycine, AMPA-kainate, and NMDA receptors. NMDA receptors are positively modulated by PS; however, the degree of modulation varies considerably.
In addition to PS, several other 3β-hydroxy steroids have been shown to potentiate NMDA receptors (see, e.g., Paul et al., J. Pharm. and Exp. Ther. 1994, 271, 677-682). Recently, a 3β-hydroxy-ergost-5-ene steroid derivative (1) was reported as positive modulator of NMDA (NR1a/NR2A). Compound (1) (also referred to as Org-1) was found to selectively modulate NMDA over GABAA (see, e.g., Madau et al., Program No. 613.2/B87. 2009 Neuroscience Meeting Planner. Chicago, Ill.: Society for Neuroscience, 2009; Connick et al., Program No. 613.1/B86. 2009 Neuroscience Meeting Planner. Chicago, Ill.: Society for Neuroscience, 2009).

New and improved neuroactive steroids are needed that modulate brain excitability for the prevention and treatment of CNS-related conditions. The compounds, compositions, and methods described herein are directed toward this end.